1. Technical Field of the Invention
The present invention relates to a method for producing light transmitting plate. More specifically, the present invention relates to a method for producing a large-sized light transmitting plate having a diagonal length of not less than 14 inches too.
2. Description of Related Art
Light transmitting plates are used as an optical element for transmitting light from light source(s) provided on the lateral sides thereof in a liquid crystal display for note-book type personal computers, desk top personal computers, television set with a liquid crystal display, etc. Arrangements of a liquid crystal display and light transmitting plates are shown in a schematic sectional view in FIG. 1. A back light unit provided in the rear side of liquid crystal display 1 is composed mainly of alight transmitting plate 2 or 3, a reflection layer 4 placed in the rear side of light transmitting plate 2 or 3, a light diffusion layer 5 facing the light transmitting plates 2 or 3 (facing the liquid crystal display), a light source 7 placed on the lateral sides of the light transmitting plate 2 or 3 and a reflector 8 for transmitting light from the light source into the light transmitting plate 2 or 3. The incident light beams from the light source 7 are reflected on the surface of the reflector 8 and enters into the light transmitting plates 2, 3 and, while passing therethrough, are reflected by the reflection layer 4 and emitted out of the front side. On the front side of the plate, light are emitted uniformly from the whole area due to the presence of the light diffusion layer 5 and serves as illumination for the liquid crystal display 1. A cold cathode-ray tube is generally used as the light source 7.
As the reflection layer 4, a reflection plate may be used. Alternatively, a pattern having a reflective function may be printed on the rear side of the light transmitting plates 2 or 3 providing the same function. As the light diffusion layer 5, a light diffusion plate may be used. Alternatively, a pattern having a light diffusion function may be printed on the front side of the light transmitting plate 2 or 3 to afford the same diffusion effect as above. Also, an application of a prism sheet as a light diffusion layer is well known.
FIG. 1 (a) shows an arrangement of a liquid crystal display and a light transmitting plate used in relatively small-sized displays with a diagonal length of not larger than about 14 inches for note-book type personal computers etc. The light transmitting plate 2 is in a shape of a wedge with the thickness gradually increasing from about 0.6 mm to about 3.5 mm. In a light transmitting plate 2 in the shape of a wedge, the light source 7 is placed at the thicker end of the plate 2. In the example shown FIG. 1 (a), one light source 7 is placed. Instead, a plural of light sources may be used. The arrangement shown in FIG. 1 (b), on the other hand, is for large-sized display in desk-top personal computer, television set with a liquid crystal display, etc. The light transmitting plate 3 is in the form of a sheet with an almost uniform thickness. In such a sheet-formed light transmitting plate 3, light sources 7, 7 are placed on two opposed lateral sides respectively. In the example shown in FIG. 1 (b), two light sources 7, 7 are placed, on each of lateral side respectively. Instead, a plurality of light sources 7,7, that is, two, three or more light sources may be disposed on each lateral side for still larger displays.
Such light transmitting plates 2, 3 are made of a methacrylate resin with a high light transmittance. The light transmitting plate 2 in the form of a wedge as shown in FIG. 1 (a) is made by injection molding while the light transmitting plate 3 in the form of a sheet as shown in FIG. 1 (b) is cut out from a resin sheet. Attempts have been made to make a light transmitting plate without printing. That is, in case of a light transmitting plate 2, which is made by injection molding, such patterns as dots or lines are added on the surface of the mold so as to form the patterns on the surface of a molded light transmitting plate that serves as reflection layer. Furthermore, this technique is applied to form the front, on a light emission surface as well, through forming a pattern with light diffusion and light orientation capabilities so as to eliminate the use of a diffusion plate or prism sheet.
A known injection molding process will be explained briefly. The injection molding equipment used includes a mold, a clamping device to drive the mold toward clamping or closing directions and an injection equipment to inject a molten resin into the mold. The mold is made up of a movable plate(s) and a stationary plate(s). In the stationary plate(s), a sprue is formed through which a molten resin is passed. A runner and gate are formed along the parting line between the movable plate(s) and the stationary plate(s), and a cavity for molding a product is formed between the movable and stationary plates. The movable plate(s) is provided with ejection means for taking out a molded product. The injection equipment is to plasticize and melt a resinous material and inject and fill the molten material into the mold cavity swiftly. The injection equipment includes a cylinder, a screw so provided therein as to be progressed by rotation and driven, a nozzle mounted at the tip of the cylinder, a hopper to feed the material resin into the cylinder, a motor to drive the screw, and a ram mechanism to drive the screw forward.
The circumferential portion of the cylinder is provided with a heater to melt the resin inside. Driven by the motor, the screw feeds the resin into the cylinder. With the heater energized, the resin is heated and compressed, and melted and kneaded, then sent to the tip of the screw and accumulated there. Then by the ram mechanism, the screw is driven forward with no rotation so as to inject the molten resin into the cavity of the mold through the nozzle at a stroke. Thus, a molded product is obtained. In the usual injection molding, the injection rate is about 20 to about 300 cm3/sec.
A series of steps to obtain a molded product comprises feeding the measured amount of resin into the cylinder, accumulating a predetermined quantity of molten resin at the tip of the cylinder, injecting and filling the molten resin into the cavity by moving the screw forward, applying an additional holding pressure to compensate for volume shrinkage caused by cooling and solidification of the molten resin, followed by cooling the molded product within the mold and measuring the molten resin for successive molding operation, moving the movable components and opening the mold to take out the molded product after cooling.
To make a light transmitting plate with a diagonal length of not less than 14 inches by above described injection molding process, it becomes necessary to utilize a larger molding equipment than usual having corresponding higher clamping strength. Further, as the product size becomes larger, the distance from the molding gate to the flowing peripheries increases and an accurate molding becomes difficult That is, in the known injection molding, a short-shot (areas where the resin failed to reach) occurs. Though volume shrinkage of molten material upon cooling which is generally compensated by holding pressure, the compensation for the pressure depression does not work effectively when the material flow paths are distant from the gate, and consequently sink marks (sunk areas caused by volume shrinkage) or inaccurately transferred patterns by the inner surface of the wall of the cavity are observed on the surf ace of the molded product. It is difficult, therefore, to obtain uniform light transmitting plate and since the light beams irradiated from the cold cathode-ray tube as a light source f ails to cover the whole area of the plate with a sufficient luminous intensity, a large-sized light transmitting plate having a diagonal length of not less than 14 inches with even thickness has not been manufactured for practical application by the injection molding process, but been made by cutting a sheet of methacrylic resins.
In other words, a light transmitting plate having a diagonal length of not less than 14 inches and still larger ones having a diagonal length of 15 inches or more are made by cutting out a uniform methacrylate resin sheet to a desired size, and a total of two, four or six cold cathode-ray tubes are disposed on the both lateral sides respectively as back light sources. Methacrylate resin sheets of about 5 to about 15 mm thick are used. In this method, the methacrylate resin sheet is first roughly cut, and then subjected to a finishing cut by means of laser-cutting process which also includes finishes of the end portions. A reflection pattern is then printed on one side of the sheet to obtain a finished product.
A problem raised by the above described cutting out method is that the sheet of methacrylate resin lacks high degree of precision in thickness and this may cause uneven printing of patterns on the surface of the sheet, and possible occurrence of open spaces between cut out sheets and frames of the product plate or fitting failure may be encountered. In the laser cutting process, furthermore, edge portions of the sheet tends to droop due to heat generated by laser beams utilized and this may lead to product failure. In addition, the printing costs in the subsequent procedure are high. The conventional method presents such problems that would not have been encountered if the light transmitting plate were made by injection molding process. Meanwhile, large light transmitting plate having a diagonal length of not less than 14 inches are not readily made by injection molding without blemish. It is also not easy to transfer desired patterns providing functions such as reflection and diffusion of light onto the surface of molded resinous product during its stay in the mold cavity due to unusual product size and inherent inferior transferability.
In view of the prior art described above, including the disadvantages and deficiencies of the prior art, the inventors conducted intensive research and succeeded in finding a method for producing a light transmitting plate, especially larger ones with a diagonal length of not less than 14 inches (355 mm), which are molded from a molten resin and which are uniform in thickness and fully meet the requirements of light transmitting plates and can configure a reflection layer pattern or light diffusion layer pattern concurrently.
It is, therefore, an object of the present invention to produce light transmitting plates including larger ones having a diagonal length of not less than 14 inches by molding from a molten resin, the plate being excellent in thickness uniformity, dimensional stability, overall production costs in a method which also forms a pattern to serve as a reflection layer or a light diffusion layer on the emission side so that the subsequent printing procedure can be eliminated.
That is, the present invention provides a method for producing a light transmitting plate, comprising the steps of:
using a molding equipment composed of an injection equipment and a mold for producing a light transmitting plate wherein a cavity of the mold communicates with a cylinder in the injection equipment;
feeding a transparent resin into the cylinder;
melting the transparent resin in the cylinder; and
injecting the molten resin into the cavity of the mold from the cylinder;
wherein a viscosity of the molten resin at the inlet of the mold is about 50 to about 5,000 Paxc2x7sec and an injection rate of the molten resin is about 1 to about 15 cm3/sec.
Also, according to the method of the present invention, it is possible to produce a light transmitting plate with high precision using a mold with functional pattern(s) provided on at least one of the surface. The resulting light transmitting plate obtained has a pattern transferred on at least one side of the surface thereof; a reflection layer pattern on the rear side thereof or a light diffusion pattern on the light emission side. Those patterns are configured based on a concave/convex pattern on the surface of the mold.
In the method of the present invention, the screw is driven forward in the cylinder or the screw is rotated to cause the molten resin continuously to flow into the mold cavity at a very low velocity to mold the resin. In the present invention, this mold method is adopted for making such light transmitting plate, especially for large ones. In this inventive method, an engraved pattern is provided at least on one cavity surface of the mold. According to this method, it is possible to mold a molten resin directly into an light transmitting plate that is excellent in thickness uniformity, dimensional stability and which is provided with a reflection layer or light diffusion layer pattern or both. That leads to reduction in overall manufacturing costs as well.